i have a mazda3 and sure its not as comfortable as the new golf and maybe a little more noisey inside, but i prefer the plastics on the interior the new golf. Also its got a very capable chassis, even in the 1.6TS2 form. Can't wait to see the higher powered version. Also, its not as dull looking as the MKV (no offence to you all who one, but it is bland).
But the biggest reason for choosing the mazda is that it has much better customer service, better value car. If i could have stretched to the Golf then I wouldn't have due to the crap customer care.
Its all friendly competition, though I doubt the mazda will be able to beat the Golf, (its not as refined, but that much less, and lacks some of the more gadget type features that the golf has as standard across the range -though the mazda very well specced at the price).
sorry for the long post... can't wait for the return of the midsize hot hatches now.